A system aiming at improving frequency use efficiency, such as WiMAX (Worldwide Interoperability for Microwave Access), LTE (Long Term Evolution) or XGP (eXtended Global Platform), employs adaptive modulation changing control in which an optimal modulation scheme is used according to circumstances.
In adaptive modulation changing control, a change of modulation scheme is generally determined by means of error rate. Specifically, if no error occurs under a modulation scheme currently used, a change is made to a modulation scheme at the next upper level (see PTD 1 (Japanese Patent Laying-Open No. 2010-219949)).